An IgGl (lambda) protein which showed a unique susceptibility towards papain digestion was isolated from the serum of a patient (Mot) with multiple myeloma. The Fab fragments of this protein were degraded rapidly into smaller peptides via an Fb fragment [Gall & D'Eustachio (1972), Biochemistry 11, 4621-4628], which corresponded to the constant domains (Cl-Chl). Structural analysis of the isolated Fab fragment, which consisted of the intact L-chain, a 17,000 and a 5000 mol. wt peptide fragment, indicated that the initial cleavage site was located in the vicinity of the second hypervariable region of the Fd fragment. Examination of the partial amino acid sequences of the Mot H-chain suggested that the variable region of the H-chain may be a hitherto unknown hybrid of subgroups I and III. This particular structure seems to have made the Fab fragment highly susceptible to papain. In the course of the present study, we also found in the papain digests of several human IgG proteins an 'intermediate' 5S fragment, which had previously been reported exclusively for the papain digest of rabbit IgG.